Please Note: The article below is a very opinionated piece - I offer it only for the interesting comment made (in bold) that speaks to the OP's question.
"The Complete Book of Numbers" - David Plotz (Slate magazine)
Chap. 31...Moses dispatches his army, which quickly kills the five
Midianite kings and slaughters all the Midianite men. ... What is
particularly poignant is that Moses himself seems to know that this
massacre of innocents is wrong. He orders his death squads to stay
outside of camp after they finish their butchery. They need a week
away from the Tabernacle to purify themselves. The Bible never
mentions such a quarantine for Israelite soldiers after other battles.
But, as Moses recognizes, these killings are not war, they are murder,
and they defile his people.
Num. 31:6 - And Moses(4872) sent(7971) them to the war,(6635) a
thousand(505) of tribe(4294) them, and Phinehas(6372) the
son(1121) of Eleazar(499) the priest,(3548) to the war,(6635) with the
holy(6944) instruments,(3627) and the trumpets(2689) to blow(8643) in
Num. 31:7 - And they warred(6633) against(5921)
the Midianites,(4080) as(834) the LORD(3068) commanded(6680) (853)
Moses;(4872) and they slew(2026) all(3605) the males.(2145)
Moses wasn't wroth until they returned with dreaded captives; otherwise, perhaps they wouldn't have needed the purification that followed, because the war was commanded by Yahweh.
After the slaughter at Shechem, Jacob was wringing his hands and expecting judgment/calamity. However, after he heard from Elohim, Jacob told the people to 'clean up' (as in take a bath?) and change their clothes. After what follows this scene (going to Beth-El), it almost looks like Elohim may have condoned the slaughter at Shechem as 'Godly zeal' for the defiling of Dinah.
Gen. 35:2 - Then Jacob(3290) said(559) unto(413) his
household,(1004) and to(413) all(3605) that(834) with(5973)
him, Put away(5493) (853) the strange(5236) gods(430) that(834)
among(8432) you, and be clean,(2891) and change(2498) your
Anyway, interesting question by OP. Still looking for mention of this subtle distinction in commentaries.
Side Note: One article mentioned that misunderstood "godly zeal" caused atrocities as the Crusades and other 'cleansings' in His name.